Process and apparatus for the isolation of pure, or substantially pure, organometallic compounds

ABSTRACT

A process and apparatus to enable the continuous isolation of an organometallic compound, such as trimethylindium form a liquid feedstock. The liquid feedstock is delivered to a distillation column ( 2 ) having two heating zones ( 6, 8 ) to effect dissociation of the feedstock thereby liberating the organometallic compound which is collected as a vapour form the top ( 4 ) of the column.

[0001] The present invention relates to an improved process andapparatus for the isolation of pure, or substantially pure,organometallic compounds, in particular organo compounds of Group 3ametals, especially Trimethylindium.

[0002] Organometallic compounds, such as Trimethylindium,Trimethylgallium and Triethylgallium are commonly employed as metalsources in the fields of epitaxial semiconductor growth and/orprocessing, vapour or plasma etching, plasma deposition or thin filmdeposition, for example Metalorganic Chemical Vapour Deposition (MOCVD).The compounds are generally isolated using a batch process wherein thereactants are fed into a vessel and the product collected under thecorrect experimental conditions. Organometallic compounds are usuallyisolated from a liquid feedstock, which is formed in the productionprocess, comprising the group 3a compound and a suitable ligand, forexample tetraglyme. Thermal dissociation of this feedstock liberates thedesired product under mild conditions and leaves the less volatilecomponents in the main vessel. This process provides compounds that aresufficiently pure but requires the use of large reaction vessels toproduce sufficient yield of product. The production rate of the compoundis also limited due to the necessity to charge the main vessel, collectthe product and then clean out the vessel before commencing the processagain. Cleaning equipment between runs may also lead to increasedcontamination of the product.

[0003] U.S. Pat. No. 5,336,473 (Nadler, K et al) describes an apparatusfor removing cobalt values from a crude product using a stripper reactorwherein the crude product is fed to the top of the reactor. Such anarrangement would not be suitable for continuously isolatingorganometallic compounds such as trimethylindium.

[0004] It is an aim of the present invention to provide an improvedprocess and apparatus for the isolation of pure, or substantially pure,organometallic compounds, in particular the isolation ofTrimethylindium, which overcome the above-mentioned drawbacks.

[0005] Accordingly, a first aspect of the present invention provides acontinuous process for the isolation of an organometallic compoundcomprising the steps of delivering a liquid feedstock containing thedesired organometallic compound to a heated reaction centre of adistillation column, allowing thermal dissociation of the feedstock tooccur in a first heating zone at or below the heated reaction centre toliberate the organometallic compound in the vapour phase, passing theorganometallic compound through a second heating zone above the heatedreaction centre to maintain it in the vapour phase and collecting theisolated organometallic compound.

[0006] A second aspect of the present invention provides an apparatusfor the continuous isolation of a substantially pure organometalliccompound, the apparatus comprising a delivery conduit for input of aliquid feedstock, a distillation column and at least one outlet, thedistillation column having at least one controllable heat source toprovide a first heating zone at or below the centre of the column, suchas to cause thermal dissociation of the feedstock and vapourisation ofthe product and a second heating zone above the centre of the column toprevent condensation of the vapourised product thereby enablingcontinuous collection of the vaporised product through the outlet.

[0007] The process of the present invention may be used to isolate anyorganometallic compound from a feedstock wherein the product is volatileand the by-products are less volatile.

[0008] The process requires the feedstock to be fed into a distillationcolumn, preferably into the centre of the column. An outlet ispreferably provided at or near the top of the column for the collectionof the isolated product and a further outlet is preferably provided ator near the base of the column for removal of waste products. Acollection vessel, connected to the first outlet, is preferably providedfor collection of the product. Preferably, a second or furthercollection vessel, connected to the second outlet, is provided forcollection of waste products.

[0009] The first collection vessel is preferably cooled prior tocommencing the process. More preferably, the vessel is cooled to atleast −15° C., more preferably at least −20° C.

[0010] The equipment is maintained at specified temperatures to controlthe rate of removal of the product from the top of the column. Thecontinuous delivery of the feedstock at a predetermined rate into thecolumn, together with the maintenance of a specified temperature andpressure differential in the column enables a continuous production ofthe pure organoometallic compound to be achieved. The rate of additionof the feedstock may be controlled by means of appropriate flowcontrollers, preferably liquid mass flow controllers.

[0011] The process is preferably carried out under vacuum, preferably ata pressure of at least 1 Torr (133.32 Nm⁻²), more preferably at least 2Torr (266.64 Nm⁻²).

[0012] The continuous process relies on an equilibrium being set up inthe column such that the product goes up the column and waste productsmove down. A number of factors effect the maintenance of thisequilibrium, such as temperature of the column, the system pressure, theaddition rate of the feedstock, and the removal rate of material fromthe column. The conditions employed will vary according to the feedstockcomposition and the organometallic compound being collected. Preferablythe temperature and/or pressure will remain fixed with the main controlbeing the addition rate of the feedstock.

[0013] The distillation column for use in the process and apparatus ofthe present invention is equipped with two heating zones, a firstheating zone to dissociate the liquid feedstock and liberate the desiredorganometallic compound and a second heating zone to ensure theorganometallic remains in the vapour phase and the other components arecondensed. The temperatures of the respective heating zones will dependupon the feedstock being used and the organometallic compound to beisolated.

[0014] Any suitable distillation column may be used for carrying out theprocess of the present invention, such as packed or plate distillationcolumns. The column should be equipped with sufficient plates in thedecomposition section to ensure maximum removal of product from thefeedstock and sufficient plates in the rectification section to ensurepure product leaves the top of the column.

[0015] The reaction centre is preferably heated prior to commencingdelivery of the feedstock in order to achieve efficient productseparation and a steady state. Collection of product may be commencedimmediately on addition of the feedstock but preferably is commencedafter a constant temperature in the rectification column has beenachieved. Preferably, collection of product is halted if a rise intemperature in the rectification column is observed indicating lessvolatile component boil up.

[0016] The process of the present invention is particularly suitable forthe preparation of Trimethylindium in a continuous fashion. Preferably,the feedstock is the adduct trimethylindium tetraglyme. The ratio oftrimethylindium tetraglyme is preferably in the range 2:1 to 3:1. Thelower heating zone is preferably heated to 100° C.-140° C., morepreferably 120° C. to 130° C. to achieve cracking of the adduct and tovapourise the trimethylindium. The upper heating zone is preferablyheated to 30° C. to 60° C., more preferably at least 40° C. to preventcondensation of trimethylindium as it passes up the column.

[0017] The present invention will now be further illustrated by means ofthe following Examples which describe the continuous preparation ofTrimethylindium using the process and apparatus of the present inventionand with reference to the accompanying drawing which is a schematicdiagram of one embodiment of the present invention.

EXAMPLE 1

[0018] A main distillation column 2 was packed and placed under inertatmosphere using a series of vacuum/nitrogen cycles passed through theupper outlet 4 at the top of the column. A lower heated zone 6 wasraised in temperature to 120° C. and a upper heated zone 8 raised to 60°C. The initial product collection vessel (not shown) was cooled to −20°C. A vacuum was then established in the system through the collectionvessel of approximately 1 Torr (133.32 Nm⁻²).

[0019] Once a steady state had been achieved, dosing of the feedstockthrough inlet 10 into the centre 12 of the column was begun at a rate of7 ml/min. The feedstock was an adduct of Trimethylindium (TMI) andTetraglyme in a ratio 2.25:1. Solid TMI was observed to be collected inthe collection vessel, via the outlet 4, after several minutes once aliquid equilibrium had been established in the column.

[0020] After 3 hours, approximately 1.25 litres of feedstock had beenadded to the column with 500 g of solid TMI product collected from thetop of the column. The waste product, a reduced ratio TMI. Tetraglymeadduct, was removed from the bottom of the column into a separate vessel(not shown), via a lower outlet 14.

[0021] As the 1:1 TMI. Tetraglyme adduct is thermally stable and thuswill not liberate further TMI the process efficiency was calculated onthe free TMI present in the feedstock and proved to be 84%

EXAMPLE 2

[0022] The apparatus used in Example 1 above was employed withoutcleaning. Hence, the column was primed with feedstock/waste productsfrom the start. The lower heated zone 6 was again raised in temperatureto 120° C. but the upper heated zone 8 was raised to 40° C. All otherprocedures were the same except a vacuum of approximately 2.4 Torr(319.968 Nm²) was established in the system through the collectionvessel and dosing of the TMI. Tetraglyme feedstock was begun at a rateof 4 ml/min in a ratio 2.3:1. Solid TMI was observed to be collectedalmost immediately as the liquid equilibrium in the column wasestablished.

[0023] After 6.5 hours approximately 1.5 litres of feedstock had beenadded to the column with 450 g of solid TMI product collected from thetop of the column. The waste product, a reduced ratio TMI. Tetraglymeadduct, was removed from the bottom of the column.

[0024] As the 1:1 TMI. Tetraglyme adduct is thermally stable and thuswill not liberate further TMI the process efficiency was calculated onthe free TMI present in the feedstock and proved to be 70%.

EXAMPLE 3

[0025] The process described in Example 1 above was repeated. The lowerheated zone was again raised in temperature to 120° C. but the upperheated zone was raised to 40° C. A vacuum of approximately 2 Torr(266.64 Nm⁻²) was established and once a steady state had been achieved,dosing of the TMI. Tetraglyme feedstock (in a ratio 2.3:1) was begun ata rate of 2 ml/min. Solid TMI was observed to be collected after severalminutes once a liquid equilibrium had been established in the column.

[0026] After 8 hours approximately 1 litre of feedstock had been addedto the column with 390 g of solid TMI product collected from the top ofthe column. The waste product, a reduced ratio TMI. Tetraglyme adduct,was removed from the bottom of the column.

[0027] As the 1:1 TMI. Tetraglyme adduct is thermally stable and thuswill not liberate further TMI the process efficiency was calculated onthe free TMI present in the feedstock and proved to be 96%.

[0028] The process and apparatus of the present invention enable thecontinuous isolation of an organometallic compound to be achieved withsufficient yields of product. This allows the ready scale up of theprocess without the need for huge plant vessels and additionalprocessing steps. The continuous process allows the use of much smallervessels, for example 10 L holding vessels as opposed to 200 L vessels,to produce a similar rate of production with associated safety benefits.Additionally the process avoids the need for frequent dismantling andcleaning of the equipment thereby reducing potential contamination ofthe product and again increasing production rates and improving safetyratings. The process and apparatus are applicable to both solid andliquid products using a suitable liquid feedstock.

1. A continuous process for the isolation of an organometiallic compound comprising the steps of delivering a liquid feedstock containing the desired organometallic compound to a heated reaction center of a distillation column, allowing thermal dissociation of the feedstock to occur in a first heating zone at or below the heated reactor center to liberate the organometallic compound in the vapour phase, passing the organometallic compound through a second heating zone (8) above the heated reactor center to maintain it in the vapour phase and collecting the isolated organometallic compound.
 2. A continuous process as claimed in claim 1, wherein the isolated product is volatile and waste products produced from the dissociation are less volatile.
 3. A continuous process as clamed in claim 1 or wherein the dissociated organometallic compound is collected as a vapour at or near the top of the column.
 4. A continuous process as claimed in claim 1, wherein waste products are collected at or near the base of the column.
 5. A continuous process as clamed in in claim 1, wherein the isolated product is collected in a separate collection vessel that is cooled to at least −15°.
 6. A continuous process as claimed in claim 5 wherein the selection vessel is cooled to −20°.
 7. A continuous process as claimed in claim 1 wherein the process is maintained at specified temperatures to control the rate of removal of the product from the column.
 8. A continuous process as claimed in claim 1 wherein continuous delivery of the feedstock at a predetermined rate into the column, together with the maintenance of a specified temperature and pressure differential I the column enables a continuous production of isolated organometallic compound to be achieved.
 9. A continuous process as claimed in claim 1 wherein the dissociation and isolation is carried out under vaccuum.
 10. A continuous process as claimed in claim 9 wherein the dissociation and isolation is carried out a pressure of at least 1 Torr (133.32 Nm⁻²).
 11. A continuous process as claimed in claim 8 wherein the temperature and/or pressure is kept fixed and the main control for the isolation of the product is the addition rate of the feedstock.
 12. A continuous process as claimed in claim 1, wherein the first heating zone is kept at a higher temperature than the second heating zone.
 13. A continuous process as claimed in claim 1, wherein the reaction centre is heated prior to commencing delivery of the feedstock.
 14. A continuous process as claimed claim 1, wherein collection of the product is commenced after a constant temperature has been achieved in the column.
 15. A continuous process as claimed in claim 1 claimed for the preparation of trimethylindium.
 16. A continuous process as claimed in claim 15 wherein the feedstock is the adduct trimethylindium tetraglyme.
 17. A continuous process as claimed in claim 16 wherein the ration of trimethylindium tetraglyme is in the range 2:1 to 3:1.
 18. A continuous process as claimed in claim 15 wherein the lower heating zone is maintained at a temperature of between 100 to 140° C. and the upper heating zone is maintained at a temperature of between 30° C. and 60° C.
 19. An apparatus for the continuous isolation of a substantially pure organometallic compound, the apparatus comprising a delivery conduit for input of liquid feedstock, a distillation column and at least one outlet, the distillation column having at least one controllable heat source to provide a first heating zone at or below the center of the column such as to cause thermal dissociation of the feedstock and vapourisation of the product, and a second heating zone above the center of the column to prevent condensation of the vapourised product thereby enabling continuous collection of the vapourised product through the outlet.
 20. An apparatus as claimed in claim 19 wherein the outlet is provided at or near the top of the column for collection of the isolated product and a further outlet is provided at or near the base of the column for removal of waste product.
 21. An apparatus as claimed in claims 19 wherein a collection vessel is connected to the outlet for collection of the product.
 22. An apparatus as claimed in claim 19 wherein the liquid mass flow controllers are provided to control the rate of addition of the feedstock to the column.
 23. The use of an apparatus as claimed in claim 19 for the isolation of trimethylindium.
 24. The use of an apparatus as claimed in claim 23 wherein the feedstock is trimethylindium tetraglyme.
 25. The use of an apparatus as claimed in claim 23 wherein the first heating zone is maintained at a temperature in the range of 100° C.-140° C. and the second heating zone is maintained at a temperature in the range of 30° C. to 60° C. 